A diesel engine which uses clean-burning DME (dimethyl ether), instead of light oil, as fuel is now attracting attention as means for reducing air pollution by diesel engines. DME fuel is a liquefied gas fuel unlike light oil as a conventional fuel. That is, DME fuel has a boiling point lower than that of light oil and vaporizes at room temperature, whereas light oil exists as a liquid at atmospheric pressure and temperature. Thus, in the case that DME fuel is used in a conventional diesel engine, the DME fuel vaporizes when the supply pressure to an injection pump is low. Thus, to supply liquid DME fuel to an injection pump, the supply pressure to the injection pump must be higher than that required to deliver light oil to the injection pump.
Thus, when DME fuel is used in a conventional diesel engine, the amount of fuel leaking through a gap between a plunger barrel and a plunger of an injection pump for delivering the DME fuel to a fuel injection nozzle of the engine into a cam chamber of the injection pump is greater than the amount of fuel leaking when light oil fuel is used due to the high supply pressure to the injection pump. In addition, DME has a lower viscosity than that of light oil and thus easily leaks through the gap, which increases the amount of fuel leaking through the gap. When DME fuel in a liquid state leaked through the gap between the plunger barrel and the plunger flows into the cam chamber of the injection pump and is mixed with lubricating oil in the cam chamber, the viscosity of the lubricating oil is reduced, which causes a possibility of a malfunction of the injection pump. It is difficult to separate and remove the DME fuel in a liquid state mixed with lubricating oil from the lubricating oil, and also it takes a long time for the DME fuel to vaporize and be removed from the lubricating oil. There arises a need for the injection pump of the DME fuel supply device for a diesel engine to minimize the amount of DME fuel in a liquid state leaking through the gap between the plunger barrel and the plunger into the cam chamber.
Even if the plunger barrel and the plunger are formed with high accuracy to minimize the gap there between, however, there is a limit to reducing DME fuel leaks. Thus, an example of means for solving the problem is a DME fuel supply device for a diesel engine adapted to separate lubricating oil from vaporized DME fuel, a vapor phase part, in the cam chamber with an oil separator, pump the separated DME fuel in a vapor state, and returns it to a fuel tank. This accelerates vaporization of DME fuel in a liquid state leaked into the cam chamber so that the amount of DME fuel in a liquid state to be mixed with lubricating oil can be reduced, and also accelerates vaporization of DME fuel in a liquid state mixed with lubricating oil. Therefore, the time required for DME fuel in a liquid state to be separated from lubricating oil can be reduced. This prevents reduced lubricating performance of lubricating oil caused by DME fuel mixed with the lubricating oil.
Meanwhile, the cam chamber is held at a constant pressure which is atmospheric pressure or above by a pressure regulating valve or the like, since the entry of oxygen into the cam chamber must be prevented. Thus, smooth vaporization of DME fuel is prevented, and even if DME fuel separated by the oil separator described above is pumped and returned to the fuel tank, not all the DME fuel leaked into the cam chamber is returned, so that the amount of DME fuel mixed with lubricating oil in the cam chamber is gradually increased. This accelerates deterioration of lubricating oil in the cam chamber, resulting in reduced lubricating performance of lubricating oil in a short period of time, and lubricating oil in the cam chamber needs to be replaced at short intervals.